Atmosphere And Weather

Anatomy of a Low Pressure Area

Nancy Houser's image for:
"Anatomy of a Low Pressure Area"
Image by: 

Forecasting the weather has been a process dating back to the Babylonians in 650 BC, with weather predictions formed from cloud patterns and astrology. By 340 BC, Aristotle had begun to describe weather patterns in Meteorologica, a treatise containing personal theories on earth sciences that consisted of his earliest accounts of water evaporation, earthquakes and weather phenomena. The information today on the anatomy of a low pressure area has come a long way since then, but its importance on our daily weather patterns has become vital.

The anatomy of a low pressure area is associated with developing areas of wind divergence, forming in the upper levels of the tropospheric. The troposphere is considered the lowest region of the atmosphere, located between the surface of the earth and the tropopause where temperatures decrease as the altitude increases. The formation process is referred to as "cyclogenesis." This is the formation that develops when the atmospheric pressure at sea level becomes equal to the surrounding area.          

The low system is associated with upcoming severe weather, while a high pressure is associated with fair skies and light winds. Surface air pressure consists of the measurement of atmospheric weight in a certain area, with low pressure areas considered an area where there is less atmosphere to overly it. Low pressure areas begin to develop when the up and down movement of air in atmospheric circulations remove a certain amount of atmosphere from a specific area.

~ Consistent global pressure areas

There are no fixed locations for low pressure areas in land areas involving towns and cities. This is because they are formed in oceans and seas – built by pressure formation from water. This low pressure develops into powerful hurricanes, cyclones and typhoons. However, there are four consistent global pressures areas listed below and their characteristics:

Equatorial Low Pressure Trough – Also known as, the Inter-Tropical Convergence Zone, the inconsistent equatorial low pressure trough is a narrow zone located between two belts of the trade winds. This general atmosphere circulation occupies the area of the tropics. It blows northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere. Polar High Pressure Cells – also called the Polar Hadley Cells, the polar high pressure cells are the cold, dry prevailing winds blowing from the high pressure areas of the polar highs – the area of greatest heat. They spiral outwards from a high pressure center, with the Polar Climate hugging the Arctic coast to yield a 60-day growing season. Sub polar Low Pressure Cells – a continuous zone of low pressure in the Southern Hemisphere, located between 50 and 70 degrees.

Their intensity varies with the seasons but is the most intense during the summer. The main reason is the existence of varying temperatures of the two air masses located on either side of the zone. Subtropical High Pressure Cells – a system of numerous localized anticyclonic high pressure cells, located at 20 to 30 degrees latitude. This system develops because of the descending Hadley cell air currents, intensifying over the ocean in the summer or high Sun seasons. Air that is located over the oceans remains cool because of slow heating of the ocean water in relation to the land surfaces around it. On the land, the winter months cause the land too cool quickly while forming large cold continental air masses.

~ Characteristics of a low pressure area

When a low pressure area begins to move into a specific region, the associated air currents begin to rise. Once this occurs, the air will cool and condense into clouds and precipitation. Two types of weather develops from this situation: showers and thunderstorms ahead of a cold front, located in warm, unstable air and with the storms or weather events occurring in short durations; or snow and steady rain will begin to fall north of the warm front, which is rising up and over the cold air ahead of the warm front.

Cold air movements in the northern hemisphere move counter-clockwise around the low pressure area, with cold air located north and west of the system. In comparison, warm area is located south and east of the area. Many developing low areas in the south bring in hurricanes and nasty weather.

~ Winds and the low pressure area

The larger the difference between a high and low pressure area, the stronger the winds will become. The conflicting pressure system will pull in air from the surrounding areas toward its center. Winds play a large factor in the anatomy of a low pressure area, as the merging of the two pressure systems will cause the pressure gradient to increase. This will cause stronger winds to develop, as the wind flows from a high pressure to a low pressure area.

What influences this is the rotation of the Earth, causing the air to deflect to the Northern Hemisphere or the Southern Hemisphere instead of directly left and right. This causes the wind to flow "around the high and low pressure areas" in large and long-lived pressure systems. However, an exception to this would be something like a small thunderstorm where the wind actually does flow directly from high pressure to low pressure.

More about this author: Nancy Houser

From Around the Web